The demands on the reliability of computer systems are extremely high. A system which can continue its normal system operation even when power has failed due to external factors such as a breakdown of the power facilities of the computer system or lightning, a system which can complete the processing currently in progress normally even when the power has failed for more than an allowable extent, and a system which can guarantee the safety of the data being written into the magnetic disk apparatus are demanded.
Therefore, even in a magnetic disk apparatus provided as a subsystem of a computer, it is necessary to control the power to enable efficient and inexpensive back-up when the input power has failed.
In general, for example, a 60 to 360 Gbyte magnetic disk apparatus connected to a large-sized computer system installed in a computer center is provided with a full-scale power facility along with the large-sized computer system, so no battery is provided in the magnetic disk apparatus itself.
That is, the large-sized computer system and the magnetic disk apparatus receive the supply of power from a common power facility. In this case, the power facility is provided with an external power supply and a back-up battery and further is provided with an emergency generator in some cases. In general, the back-up battery is large in capacity and therefore there are various restrictions as to the construction and appearance of the battery due to provisions of fire prevention laws, so the installation space becomes large.
On the other hand, a medium-sized computer system installed in a general office etc. uses a medium-sized magnetic disk apparatus of for example a 5 to 20 Gbyte capacity. In the case of such a medium-sized computer system, there is no full-scale power facility provided as in the case of a large-sized computer system. Rather, the commercial power is used. Therefore, it is necessary to provide a back-up battery for the magnetic disk apparatus in this case.
In a medium-sized computer system, however, when providing a back-up battery in the magnetic disk apparatus, it is necessary to make the installation space of the battery as small as possible and also to limit the battery to the range of power capacity which is free from restriction under fire prevention laws. On the other hand, no matter how small the battery capacity, back-up power is required and must be guaranteed. In this case, the consumption (discharge) of the battery when power fails or is momentarily cut off is remarkable. If the battery cannot be charged fast enough, then there is the danger that back-up of power can no longer be guaranteed for the system.
Accordingly, in a magnetic disk apparatus used for a medium-sized computer system, a key problem is how to suppress the consumption of the back-up battery as much as possible.
On the other hand, a magnetic disk apparatus carries a plurality of compact magnetic disk modules in a single locker. If all the modules are activated at once, then current of a level several times the steady state flows and a large capacity of power becomes required. Accordingly, the modules are activated in succession so as prevent the rush current from overlapping. To achieve further compactness of the power facilities, however, it is desired to control the activation even finer.
Further, improvements are required in the monitoring of the power, the battery test, the analysis of the causes at the time of power failure, the display for maintenance of the power, etc.